As optical component technology has improved, it has become possible to increase the traffic sent over an optical fiber by sending multiple signals, each on its own wavelength, rather than increasing the rate of a single signal. Such multiplexing is referred to as wavelength-division multiplexing (WDM). To accommodate the advances in optical technology, a new standard has been developed, called the Optical Transport Network (OTN). This standard, sometimes referred to as G.709, is designed to transport data packet traffic such as Internet Protocol (IP) and Ethernet traffic over optical fibers, as well as to transport Synchronous Optical Network (SONET) and Synchronous Digital hierarchy (SDH) traffic. It is also sometimes called digital wrapper technology because it wraps any client signal with overhead information for operations, administration, and management.
One reason for developing a new signal format for WDM signals (instead of just using the existing SONET/SDH signals) was the possibility to add new overhead channels that would give the added functionality required to efficiently perform Operations, Administration, Maintenance and Provisioning (OAM&P) on the WDM network. Another reason for developing a new standard was to provide a means for more powerful forward error correction (FEC) capability. In contrast, a relatively modest FEC capability was added to SONET/SDH. As signals traverse a multi-hop optical network, however, the signal to noise ratio decreases. Since carriers hoped to increase the transmission distances and the bit rates per wavelength, the SONET/SDH FEC was not adequate. Finally, another reason for new standards for transport was to provide a less granular payload envelope for the transport of higher bandwidth individual clients aggregated from access networks.
In an Optical Transport Unit (OTU) frame, a client signal is inserted into the frame payload area, which, together with some overhead channels, becomes the Optical Payload Unit (OPU). An OPU is conceptually similar to a SONET/SDH Path. OAM overhead is then added to the OPU to create the Optical Data Unit (ODU), which is functionally analogous to the SONET Line (SDH Multiplex Section). Transport overhead (e.g., frame alignment overhead) is then added to create an Optical Transport Unit (OTU), which is the fully formatted digital signal and functionally analogous to the SONET Section (SDH Regenerator Section). The OTU is then transmitted on a wavelength, which constitutes the Optical Channel (OCh).
There are four currently defined OTU rates and five OPU rates. Each unit has a different frame period as shown in the following table.
OPUk payloadOTUk/ODUk/OPUkkOTUk signal ratearea rateframe period0Not applicable238/239 × 1 244 16098.354 μskbit/s = 1 238 954kbit/s1255/238 × 2 488 3202 488 320 kbit/s48.971 μskbit/s = 2 666 057kbit/s2255/237 × 9 953 280238/237 × 9 953 28012.191 μskbit/s = 10 709 225kbit/s = 9 995 277kbit/skbit/s3255/236 × 39 813 120238/236 × 39 813 120 3.035 μskbit/s = 43 018 414kbit/s = 40 150 519kbit/skbit/s4255/227 × 99 532 800238/227 × 99 532 800 1.168 μskbit/s = 111 809 974kbit/s = 104 355 975kbit/skbit/sNote:All rates are ±20 ppm.
The OTU consists of the ODU, the OTU overhead, and a Forward Error Correction (FEC) code, if used. Four ODU-1s can be multiplexed into an OPU-2. An OPU-3 can contain a multiplexing of four ODU-2s, 16 ODU-1s, or a mixture of ODU-1s and ODU-2s.
FIG. 1 shows a block diagram 100 of a mapper and switch that maps a SONET OC192 signal at 9.953280 kilobits per second (kbps) to an OTN signal at 255/237 times 9.953280 kbps. The ingress card 102 receives the incoming SONET signal from an optical fiber and buffers it as needed due to the bursty nature of the synchronous mapper 104. The synchronous mapper 104 wraps the payload of the SONET signal with an ODU overhead to form an ODU-2 signal that is 239/237 times faster in kbps than the incoming SONET signal. The cross connect 106 routes the ODU-2 signal to a particular egress card 108. The egress card converts the ODU-2 signal to the desired OTN signal for transmission on an optical fiber. The reverse of the operations depicted in FIG. 1 are implemented when de-mapping an OTN signal to a SONET signal.
The ingress card 102 has a clock recovery circuit that detects a clock rate of the incoming signal. The cross connect 106 has a highly precise reference clock of its own. The egress card 108 also has its own clock. A synchronization problem arises due to the difference between the clock rate of the ingress card and the clock rate of the egress card. In particular, when the two clocks are not synchronized, loss of information may occur. Thus, it is desirable to have synchronization of the ingress and egress clocks to the reference clock to prevent a loss of information. This is true in the case of de-mapping as well as mapping.